The detection of certain biological molecules plays an important part in many aspects of life. For example, in the medical field, there is an ever-present need to detect bacterial or viral pathogens, or biological molecules. Other fields in which sensitive assays are essential include the food and beverage industries. One method of detection involves the use of electrochemically active compounds. The application of electrochemical detection has a number of advantages over other methods, such as fluorescent detection. Electrochemical detection has the potential for very high levels of sensitivity and exhibits a wider linear dynamic range than fluorescence. Furthermore, there is no requirement for samples to be optically clear. There is also less interference from background contaminants (many biological samples auto-fluoresce).
WO03/074731 discloses electrochemically active markers and methods of probing for a nucleic acid. The methods involve contacting a nucleic acid solution with an oligonucleotide probe attached to an electrochemically active marker. The probe is caused to at least partially hybridise with any complementary target sequence which may be present in the nucleic acid solution. Following enzymatic degradation of the nucleic acid probe, information is electrochemically determined relating to the marker.
Hillier et al (Bioelectrochemistry 63 (2004) 307-310) describes the use of ferrocene urea compounds as labels in pulse electrochemical methods for the electrochemical discrimination between a labelled oligonucleotide and an enzyme digested labelled oligonucleotide.
WO2005/005657 discloses further electrochemically active markers and methods of detecting protease activity. The methods involve contacting a sample solution with a protease substrate attached to an electrochemically active marker, providing conditions under which any protease present in the sample can degrade the protease substrate. Following degradation, information is electrochemically determined relating to the marker.
WO2012/085591 and WO2013/190328 describe certain diferrocenyl compounds for use as electrochemical labels.
There is a continuing need to develop labels that enable detection of the presence of biological substrates or indicators, for example, nucleic acids or amino acids, in low concentrations. In particular, there is a continuing need for new labels with different oxidation potentials and/or with different chemical or physical properties thereby widening the range of possible assays available and increasing the scope for the development of multiplex reactions. Furthermore there is a need for electrochemically active compounds which can be used as internal controls in assays. Such compounds need to give robust, consistent electrochemical responses.